Snorkeling works best for guests who are comfortable floating, using a mask, and staying near a guide. It gives you that close reef feeling without the training step of scuba, but it still asks you to be relaxed in the water.
A glass-bottom boat is better for guests who want to see the reef while staying dry, or for families where one person wants the ocean view without the physical effort. It is also a smart add-on when a group wants one shared experience after separate activities.